ES-6 introduced the support for immutable variables or constant variables. Constant variables are not a new concept in programming and have been in existence in most of the programming languages of today.
If while programming you come across a scenario where you don’t want somebody to change value of a variable then it makes sense to declare the variable as constant. You must understand that you cannot reassign a constant variable to a new value but in case of objects you can still be able to modify the property of objects.
Some rules accompanying a constant variable:
a.) Must be declared using const keyword
b.) Must be initialized at time of declaration
c.) Cannot change value of const once assigned
Let’s try to understand with examples

 <script type="text/javascript">
            const  message="This is some message";

            //message = "This is not allowed";

            const someObj = {id:1 , name :"test"};

            //This is not allowed
            //someObj = {id:2 , name :"test"}; 

            //This is allowed
            someObj.id =2;

            console.log(someObj.id);
        </script>

Constant variable also has blocked scope just like let keyword explained in previous articles.

<script type="text/javascript">
           function testContantScope(index){
               if (index == 1){
                const somedata = "this is a test";
                console.log(somedata);
               }

               console.log(somedata);
           }

           testContantScope(10);
        </script>

Outputs:
this is a test
SCRIPT5009: ‘somedata’ is undefined